The present invention relates to a rudder assembly for a ship. More particularly this invention concerns a multiple-rudder assembly of the so-called balanced type.
A balanced-type ship rudder assembly is known having a pair of like rudders pivotal about respective upright axes spaced symmetrically to opposite sides of the keel plane of the ship. Means is provided on the ship for generating a screw race, that is a backwardly flowing column of water, which runs along the plane between and around the two rudders. Each rudder has a central plane which extends parallel to the keel axis when the ship is being displaced straight forward, and which passes through the approximate center of the body of the respective rudder. The pivot axis normally lies on this center line but may, as described in German Pat. No. 949,451, be parallel to this central plane and spaced inwardly therefrom toward the keel plane.
A rudder assembly is known (see German Pat. No. 935,835) wherein the inner faces of the rudders are generally flat. In the normal rudder position, that is the position they assume for straight-ahead travel of the ship, these inner faces converge at an angle of 5.degree.. The pivot axes of the rudders are rigidly linked together so that the central planes of the two rudders always lie at the same angle to the keel axis, this angle being 0.degree. for straight-ahead travel as described above. Thus, when the two rudders are deflected to one side for a turn, the inner faces will not lie at the same angle to the keel axis, but the one will lie at an angle 5.degree. greater than the other. Thus the screw race is deflected off the outwardly lying rudder more quickly than the inner rudder so that the advantageous sharp deflection on the outside of the rudder is considerably reduced thereby reducing the steering effect. Furthermore, when in the normal straight-ahead position the above-described 520 convergence creates a certain amount of flow resistance which causes a power loss. The outside faces of the rudders in such an assembly are normally fully convex so when these rudders are inclined to the keel axis the steering effect is reduced. Thus it is impossible with which an assembly to turn a ship in its own length, the so-called optimum turning characteristic often necessary for docking operations.
Another dual rudder balance assembly is known having assymetrical rudders (see German Petty Pat. No. 1,906,399). The leading edge of each rudder is part-cylindrical and has a center of curvature lying outside the respective central plane. Furthermore, the inner face is very convex and the outer face is flat. Thus the rudder fits well into the flow path in back of the ship. Such an arrangement has the considerable disadvantage, however, that when the rudders are turned sharply to the side there is a great deal of turbulence due to this flat outer face.
It is also known to provide a rudder system (see German published specification No. 2,303,299) having symmetrical rudders whose inner and outer faces taper backwardly at an angle of 15.degree. to a tail portion which diverges at an angle of approximately 10.degree.. In this arrangement the space between the rudders which acts as a diffuser first diverges then converges in the direction of flow so as to create a considerable amount of turbulence in a screw race passing between the rudders. Thus, energy which could otherwise be used to advance the ship through the water is transformed into this turbulence and wasted. Furthermore, the flow around the outside of the rudders is similarly turbulent so that drag is created.
Finally, a system is known (see U.S. Pat. No. 969,642) wherein the individual rudders can be rotated independently of one another about their respective axes. Thus, it is possible to swing them outwardly in opposite directions through 90.degree. from the normal positions so as effectively to form a wall across the screw race with a small gap between the leading edges of the rudders constituting a chink in this wall. It is further possible with this system to swing the rudders 23.degree. further from this position so that they lie at an obtuse angle of 113.degree. to the keel plane in order that they deflect the normally backwardly forward stream at an angle of 20.degree. forwardly. In such a system it is possible to use a single-direction propeller so that a direct-drive Diesel plant may be employed to drive the ship both forwardly and in reverse. Nevertheless the difficulty with such a system is that as the rudders are swung around to deflect the screw race forwardly the gap between them increases in a quadratic ratio to the angle over 90.degree. assumed by the rudder. Thus it is standard practice in such a system to provide a so-called Steven post in the back of the rudder. This is disadvantageous because it creates two small gaps of very narrow width, causing considerable loss of thrust. Furthermore, during normal forward travel of the ship the Steven post creates considerable drag and interferes with the rudder steering.